Method and apparatus for wet-cleaning crude gas streams, the method involving the crude gas stream flowing through one venturi throat which is sprayed with a scrubbing liquid via one periodically pulsating hybrid nozzle.
To wet-clean crude gas streams, use is often made of venturi scrubbers (U.S. Pat. No. 4,152,126 and U.S. Pat. No. 4,193,778). These consist of a flow tube with a constriction, the venturi throat, and, disposed above or in the throat, a supply means for the scrubbing liquid in the form of a pressure nozzle.
Dust can thus be bound up to particle sizes of 0.1 xcexcm. Dust removal takes place in three phases: 1. The particles impinge on the liquid surface, 2. they adhere to the liquid surface, 3. the drops of the liquid are removed.
The present invention relates to an improvement in dust removal by an improvement in phase 1. Phase 2 is unproblematic, since capture can always be assumed once the drop of liquid comes into contact with the dust grain. Phase 3 is carried out in a separate liquid separator, e.g. in a cyclone.
The collection of the grains of dust in phase 1 by the drops of liquid is effected primarily by inertial separation on high-velocity drops. The inertial separation improves with increasing relative velocity between drop and grain of dust and decreasing drop diameter.
In a venturi equipped with conventional nozzling, drops are generated which, depending on the type of nozzle (single-fluid nozzle, two-fluid nozzle or swell nozzle) and nozzle inlet pressure used, have a size range of 30-2000 xcexcm. Close to the nozzle, these drops all have the same initial velocity which, depending on the nozzle pressure, is in the range of 3-50 m/sec. In the venturi throat, these relatively large drops are broken down into extremely fine droplets, owing to the high gas acceleration and the shear flow in the venturi throat, and are turbulated by turbulence. The droplets, now small, and the large velocity scatter of the droplets, in terms of magnitude and direction relative to the gas stream, permit many dust particles to encounter a liquid surface, leading to a high collection efficiency of dust on the liquid surface. The separation of the dust-laden liquid droplets as a condensate is then carried out in a liquid separator. The collection efficiency is measured as a ratio of the level of the constituents to be collected in the clean gas after wet cleaning and in the crude gas prior to wet cleaning.
In venturi scrubbers with conventional nozzling, the collection efficiency depends on the extent to which the drops of the scrubbing liquid are atomized in the venturi throat and are turbulated with the crude gas, so that as many dust particles as possible impinge on the liquid surfaces and are collected. The shear forces and the degree of turbulation of the crude-gas/liquid mixture in the venturi throat decreases with increasing size of the venturi throat. The size of the venturi throat and the velocity of the crude gas stream define the venturi pressure drop of the crude gas stream. The pressure drop increases as the venturi throat becomes smaller and the crude gas stream becomes larger. The collection efficiency increases with increasing venturi pressure drop.
A drawback of all known venturi scrubbers is that only a high venturi pressure drop will lead to good collection efficiency. Typical venturi scrubbers are operated from pressure drops of 20-30 mbar up to 150 mbar. A high pressure drop means high energy demands to achieve the required pumping capacity for the crude gas stream.
Another drawback is that in the event of a change in the crude gas flow rate while the venturi throat is a fixed size, the change in the velocity of the crude gas will cause a change in the venturi pressure drop. DE 43 31 301 describes a tube-gap venturi scrubber which consequently has two adjustable venturi throats. The tube-gap venturi scrubber has a tube gap of approximately rectangular cross-section. Disposed downstream behind said tube gap above the scrubber sump is a displacer which extends over the entire length of the gap and is mounted so as to be translatable towards the tube gap and away from it. Between the walls of the tube gap and the displacer wall, two venturi throats running parallel are formed. The cross-sections of these two venturi throats can be adjusted by sliding the displacer. Proposed as feeder means for the scrubbing liquid are swell nozzles.
A drawback of this solution for controlling the pressure drop is that it is subject to mechanical wear and the adjustment means must be run, in particular, through the sump below the venturi throats, which results in sealing problems.
It is an object of the invention to achieve high collection efficiencies in a venturi scrubber without pressure drops or with low pressure drops and to provide a simple option for controlling the collection efficiency. Such control is necessary, in particular, in the event of retrofitting measures which lead to a higher gas flow rate.
The object of the invention is achieved by a method and an apparatus for cleaning a crude gas stream with the aid of an atomized scrubbing liquid.
In accordance with the method according to the invention, the crude gas stream is sprayed, by means of a hybrid nozzle, with the atomized scrubbing liquid and is then passed, without a pressure drop or with a low pressure drop up to 30 mbar, preferably up to 20 mbar, through one or more venturi throats. A hybrid nozzle is known per se from DE 43 15 385.
A hybrid nozzle is constantly supplied with a scrubbing liquid and a gas as an atomization aid. The liquid inlets and gas inlets are connected to a first resonance chamber, connected to which on its downstream side, via a restrictor, there is at least one further resonance chamber. The last resonance chamber, seen in the flow direction, is connected to the outlet orifice of the hybrid nozzle.
The atomization aid used can, for example, be air or inert gas.
The hybrid nozzle can adopt both the operating mode of a pressure nozzle and the operating mode of a two-fluid nozzle. The characterizing feature of the hybrid nozzle is that, if constantly supplied with a specific amount of liquid and a specific amount of air, it will not atomize these amounts of liquid and air uniformly, but instead, in a pulsating manner, continuously change its mode of operation.
In the pressure nozzle mode, drops having a relatively large mean drop diameter are produced continuously. The mean drop diameter is essentially determined by the nozzle outlet orifice size. The effective range of a drop is determined by its initial momentum. The initial velocity of the drops is the same for all drops. Owing to their higher mass, the large drops have a higher initial momentum and consequently a higher effective range. 99% of the atomized amount of liquid is formed by drops whose diameters differ from one another by a ratio of up to 1:20.
A two-fluid nozzle differs from a pressure nozzle in that it is additionally supplied with air, continuously generating drops having a small mean drop diameter, compared with the pressure nozzle. The mean drop diameter is determined by the mass flow ratio of atomizing air to liquid in the nozzle and decreases with increasing atomizing air flow rate. The effective range of a drop is determined by the momentum of the atomizing air and the transfer of this momentum to an entire drop cluster. As with the pressure nozzle, 99% of the atomized amount of liquid are formed by drops whose diameters differ from one another by a ratio of up to 1:20.
The pulsating change in the mode of operation in the case of the hybrid nozzle can occur, depending on the pulsation frequency, between the pressure nozzle mode and a two-fluid mode or between different two-fluid modes which differ in terms of the flow rate of the atomizing air supplied.
The pulsating change in mode of operation if the supply of the hybrid nozzle with compressed air and liquid is constant over time is generated in the hybrid nozzle itself owing to periodic start-up phenomena (autopulsation).
The pulsation occurs at a frequency of preferably from 5 to 70 Hz, particularly preferably from 10 to 20 Hz. The frequency is determined by the ratio of the size of the first resonance chamber in the direction of flow, behind the point at which the liquid enters the first resonance chamber, to the size of the second resonance chamber. By changing the entry point of the liquid into the first resonance chamber it is possible to vary the volume of said resonance chamber in the flow direction and thus vary the frequency. The smaller the volume of the first resonance chamber in the flow direction in relationship to the size of the second resonance chamber, the higher the pulsation frequency will be.
The pulsating discharge from the hybrid nozzle generates a wide spectrum of drop sizes and drop velocities. Even drops of the same size may have quite different velocities, very much in contrast to the conventional nozzles. Alternately, a spray cone containing coarse drops having a large mean drop diameter and a large effective range, and a spray cone containing fine drops having a small mean drop diameter and small effective range are generated. The drop spectrum generated encompasses size ratios of drops of up to 1:1000.
The proportions of liquid and air in the discharged spray jet changes periodically, at low pulsation frequencies up to about 20 Hz, between the extreme values of 0% and 100% liquid fraction. At higher pulsation frequencies, the amplitude becomes smaller, until the liquid fraction in the range of 70 Hz only changes periodically between 45% and 55%.
Because of the pulsation, no steady-state atomization mode is established, so that the spray jet at all times locally contains start-up flows. Consequently, the size reduction of the droplets and the turbulence are achieved as early as between the scrubbing liquid feeder means and the venturi throat and not as late as in the venturi throat. Thus the venturi throat size and the venturi pressure drop, which is dependent on that size, will now have virtually no effect on the collection efficiency, and the venturi scrubber can be operated without a pressure drop in the venturi throat. The venturi throat no longer serves for size reduction and turbulation of the droplets. The spectrum generated by the hybrid nozzle in terms of the sizes, the velocities, and the shapes of the droplets leads to particularly effective dust collection in the venturi throat.
The collection efficiency in a venturi throat, given constant supply of the hybrid nozzle with liquid, can be controlled via the additionally supplied amount of compressed air in the hybrid nozzle and via the pulsation frequency.
The amount of atomizing air supplied to the hybrid nozzle for a particular amount of water is proportional to the specific energy input (atomization energy) thus induced into the hybrid nozzle. By varying the amount of atomizing air fed into the hybrid nozzle it is possible to set the energy input to a value in the range of from 0.5 kWh/1000 m3 of gas to 50 kWh/1000 m3 of gas, preferably from 1 kWh/1000 m3 of gas to 30 kWh/1000 m3 of gas.
The apparatus according to the invention consists of a flow tube in which one or more venturi throats are located and of one or more hybrid nozzles which are disposed upstream of the venturi throats. The distance between the hybrid nozzles and the venturi throat(s) can be adjustable. The distance between hybrid nozzle and venturi throats can be optimized with respect to the collection efficiency achieved.
The distance between the nozzle outlet and the center of a venturi throat located downstream can be chosen such that the area of the venturi throat situated below the hybrid nozzle is covered by the spray jet of the hybrid nozzle, preferably by 110%.
In a preferred embodiment of the apparatus according to the invention, the one or more venturi throats are formed by at least two parallel cylinders, which are juxtaposed horizontally in a plane, and in which allocated to each throat there is at least one hybrid nozzle each. Particularly preferably, the venturi throats are formed by the parallel cylinders in conjunction with one or more displacers which are disposed downstream of the parallel cylinders. The displacers can be axially movable.
A measuring system at the outlet of the mist collector downstream of the venturi for collecting the dust-laden water drops can control the compressed-air flow rate and the pulsation frequency in the hybrid nozzle as a function of the collection efficiency.
An advantage of the method according to the invention is that the collection efficiency can be controlled in a simple manner by controlling the gas flow rate and the frequency in the hybrid nozzle. The risk of the venturi scrubber becoming clogged owing to a narrow venturi gap as required for high collection efficiencies using conventional venturi scrubbers does not apply. In the hybrid nozzle itself there is likewise no risk of solids accreting, owing to the pulsation.
The amount of scrubbing liquid required is distinctly reduced, compared with the conventional systems.
Surprisingly, the collection efficiency resulting from the venturi scrubber according to the invention equipped with the hybrid nozzle is distinctly higher, in comparison, than with conventional nozzling.
The venturi scrubber according to the invention equipped with a hybrid nozzle can be used for wet dedusting of dust-laden waste gas or for removing SO2 and other gaseous components from waste gases.